This invention relates to the use of the current-mode-control circuitry for a widely used class of power converter circuits.
Current mode control is described in detail in "Current-mode control, five different types, used with three basic classes of power converters: small-signal ac and large-signal dc characterization, stability requirements, and implementation of practical circuits," by Richard Redl and Nathan O. Sokal, in the PESC '85 Proceedings [IEEE Publication 85CH2117-0], pp. 771-785. This article is hereby incorporated by reference herein. The invention is needed in instances when the controller controls the magnitudes of currents flowing into and out of coupling capacitors in those power-converter circuits. In particular, the power-converter circuits which are the subject of the present invention include at least two periodically operated control switching devices which are turned on sequentially. The loads on these power-converter circuits are driven through one or more coupling capacitors. These capacitively coupled converter circuits include switching-mode dc/dc power converters and switching-mode dc/ac power inverters.
According to the teachings of the prior art, current-mode control could not be used with capacitively coupled power converters because the capacitor voltage would "run away" if one attempted to use current-mode control with this class of power converter. This run-away problem has been noted in the Unitrode Applications Handbook, 1985-86, pages 278-291. The reason for voltage run-away is as follows. The magnitudes and durations of the pulses of current into and out of the coupling capacitor nominally are kept equal by the current-mode controller. However, there is always a small unintended difference between the charges injected and withdrawn via the two (or for full-bridge converters, two pairs of) switches which drive alternately positive and negative currents into the coupling capacitor. Such differences can be produced by differences of switching times of the two switches, by differences of propagation delays in the signal paths to the two switches, or by subharmonic oscillation of the voltage-regulating loop. The small difference of charge injected and withdrawn results in a small dc current in the circuit branch whose current is being controlled. The dc current causes drift of the voltage across the coupling capacitor. The drift continues until the circuit reaches a steady-state condition which depends on the specific circuit design and circuit parameters. In practical cases, that steady-state operating condition can be far enough from the desired operating condition to be a serious detriment to the circuit operation.
The object of the present invention is to prevent the run-away of voltage across the coupling capacitor, so that the otherwise desirable current-mode control can be used with capacitively coupled power converters.